This paper deals with the assessment of seismic collapse capacities and the related collapse durations of steel momentresisting frames (MRFs). For this purpose, three prototypes of fve-story steel structures are studied. These structures are special, intermediate and ordinary moment-resisting frames with high, medium and low ductility, respectively (SMRF, IMRF and OMRF). The structures have been designed in accordance with the Iranian codes. Furthermore, the deterioration efects of strength and stifness for structural elements have been considered based on the results of reference experimental models. Next, using 22 pairs of far-feld ground motion records proposed by FEMA P695 instruction, incremental dynamic analysis is conducted up to the dynamic instability of structures. Afterward, the collapse capacities of these structures are evaluated and compared with the fragility curves. Also, progressive sidesway collapse durations of the structures are estimated under the selected ground motion records. The results show that SMRF has larger collapse capacity than those of the other frames, and can diminish collapse probability by 33.4% and 54.6% compared to IMRF and OMRF, respectively. Moreover, the results reveal that sidesway collapse duration of SMRF is greater than IMRF and OMRF.

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